The term "hollow profiles" is used herein to refer to peripherally closed structures of substantially uniform wall thickness surrounding a closed space, i.e. a tube of any selected cross section, e.g. a circular cross section as in a pipe or a polygonal cross section or some combination of polygonal and curved cross sections. It is also intended to refer to a structure wherein a space is bounded by a part having substantially uniform wall thickness, e.g. a channel with polygonal or curved sections. Hence the term is generally used to refer to any extruded structure having a substantially uniform wall thickness and hollow or partly bounding a space and distinguishing the extruded structure from a solid rod.
In the production of extruded hollow profiles from thermoplastic synthetic resins, and especially in the production of tubes and pipes, it is common to utilize an extrusion head which is provided with a housing defining a passage for the thermoplastic material and a mandrel disposed within this housing and supported therein for defining the inner contour of the extruded strand, the housing and the mandrel together forming a passage from which the strand emerges at the mouth of the extrusion head.
To support the mandrel in the housing, a holder is generally used which has the configuration of a spider, i.e. includes ribs or webs running across the flow path of the synthetic resin material and bridging the housing and the mandrel. Means can also be provided to subdivide this flow path, e.g. in the form of an additional hollow mandrel, and for allowing the two streams to merge proximal to the outlet or mouth and downstream of the mandrel.
In general, the flow cross section of the two coaxial passages on opposite sides of the annular partition will be greater than the flow cross section of the passage extending from this partition to the mouth.
Such extrusion head can be used at the downstream end of a plasticizing and masticating worm for processing the synthetic resin material to be extruded and for developing the high pressures required to drive the material through the head. Within this worm arrangement, the material may be liquefied utilizing added heat or the heat generated by the mastication operation.
Systems of this type are used for the fabrication of thermoplastic thick-wall and large diameter pipe, e.g. from hard polyvinyl chloride.
Because of the fact that the mandrel or mandrels are supported by ribs or webs which split the synthetic resin flow through the passages of the head, the synthetic resin streams emerging at the downstream side of these flow splitters do not fully merge, but rather tend to weld together in pressure-sensitive joints which are visible as markings on the finished product and which represents structural weakenings therein similar to "seams".
To avoid these structural defects, it has already been proposed to provide the head with an imbricated arrangement of such streams so that inner and outer streams overlap one another and hence the "seams" or junctions between in the streams are bridged by outer streams and vice versa.
These techniques have not proved to be fully satisfactory, especially with thick-walled pipe where both markings along seam lines or junction lines and the tendency to rupture preferentially along these lines remain.
It has also been proposed to improve the mixing of the streams after they pass the flow dividers utilizing counter-rotating screw formations of the passage walls, baffle surfaces and the like. While this may have proved to be successful with thin-wall pipe, they have yet to be satisfactorily demonstrated to have the desired results for thick-wall pipe.
It has also been proposed to provide a system in an extrusion head of the type described whereby counterpressure or back pressure (pressure resisting the forward displacement of the synthetic resin material) will be generated to induce merger of the partial streams.
Reference may be had, specifically, to German patent No. 19 40 195 which discloses such an extrusion head. In this construction, the mandrel is fixed by a holder in the housing and downstream of this holder the flow is divided into two coaxial streams each of which has a closed annular configuration and the sum of the cross sections thereof is greater than the cross section of the subsequent portion of the flow channel where the two streams merge. Because of this reduction in the flow cross section at merger, a back pressure develops which is intended to assist in the blending of the separate streams of resin.
However, even this system did not provide a satisfactory quality of pipe especially for large-diameter pipe and thick-wall pipe.
Furthermore, this system required an extrusion head of inordinate length to be satisfactory even for pipes of smaller wall thicknesses.